I do not believe you need an actual certification or league ranking of your NTRP rating.

However, I am certain that they do evaluate both your teaching AND playing skills -in other words, you have to be a good player to join the USPTA. Check the website, it should be the first thing you do.

I'm only in high school, it would help if certified people could come in.

Personally, I believe that you don't need a certified, actual league ranking. However, you need to be able to play at at least the 4.0 level. You have experience coaching, are you past that level?

You've said you've been teaching for over four years, and probably have been playing frequently before that. I'd say you'll have no problem. In case you DO need a ranking and official 4.0 classification, it shouldn't be too hard to obtain one for you.